U.S. Pat. No. 4,034,650 shows an axial piston type machine having a cylinder barrel mounted on a shaft. At one end, the shaft is centrally supported in a drive flange by means of a ball-and-socket joint. At its other end, the shaft is mounted in a pivotable barrel support. The cylinder barrel has a circular array of axial cylinder bores. Pistons with piston rods are guided in the cylinder bores. The piston rods are articulated on the drive flange through ball-and-socket joints. A disc s arranged between the cylinder barrel and the barrel support. The surface of the disc facing the barrel support is concave-spherical. The adjacent surface of the barrel support is of convex-spherical shape complementary to the shape of the surface of the disc. The disc is restrained against rotation. The surface of the disc adjacent the end face of the cylinder barrel is provided with diametrically opposite arcuate valving ports. Each of these valving ports extends through slightly less than 180.degree.. The valving ports serve to communicate the cylinder bores alternatingly with a fluid inlet and with a fluid outlet.
In the axial piston type motor of U.S. Pat. No. 4,034,650, the torque is generated at the swash plate. The cylinder barrel is coupled with the swash plate to rotate therewith. In the motor of U.S. Pat. No. 4,034,650, this coupling is effected by the appropriately shaped piston rods engaging, in certain angular positions, the inner walls of the cylinder bores.
In one embodiment of U.S. Pat. No. 4,034,650, the barrel support is pivotable relative to the drive flange about a pivot axis, which is laterally spaced from the rotary axis of the drive flange. Thus the barrel support is pivoted "off-center". This does not affect the tilting point, i.e. the point of intersection of the drive flange axis and the barrel axis. This design offers the advantage, that the dead volume in the cylinder bores, which has to be compressed during each revolution, is reduced. This, in turn, increases the efficiency of the axial piston motor.
A similar axial piston type motor is disclosed in U.S. Pat. No. 3,933,082.
Coupling the cylinder barrel with the drive flange through the piston rods results in non-uniform rotation of the cylinder barrel, if the motor does not work at maximum tilting angle. This is undesirable.
U.S. Pat. No. 3,760,692 discloses an axial piston motor of similar type, wherein the cylinder barrel is coupled with the drive flange through meshing toroidal or conical toothed members on the drive flange and the cylinder block. The toothed members mesh along the angle bisector between the drive flange axis and the cylinder block axis. Also here, the cylinder barrel s tilted about an off-center axis.
U.S. Pat. No. 3,775,981 relates to a hydrostatic transmission. The hydrostatic transmission comprises a pump driven by a prime mover and a variable stroke axial piston type motor fed by the pump. The pump has constant delivery during normal operation. The intake volume per revolution of the motor is variable by pivoting the cylinder barrel about an off-center pivot axis through an angle of more than 30.degree., such that the dead volume in the cylinders is kept as small as possible. The intake volume per revolution of the motor at maximum pivot position is a multiple of the delivery volume per revolution of the pump. The cylinder block of the motor is carried along by the drive flange again through peripherally arranged teeth in mesh in the region of the pivot axis and permitting the pivoting movement of the cylinder barrel.
U.S. Pat. No. 3,775,981 illustrates the control of such a transmission. The pressure acting in the cylinders of the pump and motor result in a torque exerted by the cylinder barrel on the barrel support about the pivot axis. This torque is counteracted by a hydraulic actuator.
The pump delivers a constant fluid flow. The pressure in the system is controlled by a pressure control device. The pressure control device comprises a valve spool which is, at one end, engaged by a compression spring and, at the other end, exposed to the system pressure. The valve spool governs communication of the hydraulic actuator to either system pressure or to a reservoir. If the pressure increases, because the motor has to overcome higher resistance, the valve spool will be moved against the action of the compression spring and temporarily communicate the hydraulic actuator to the reservoir. Thus hydraulic fluid will flow out of the actuator and the cylinder barrel under the action of the said torque and the cylinder barrel will move to a position, where the barrel axis and the drive flange axis form a larger angle. The intake volume of the motor per revolution will be increased. The motor, therefore, will rotate more slowly, and the pressure in the system wi i drop, until a balance between compression spring force and system pressure has been reached again. The power of the motor can be varied by a control lever, by which the bias of the compression spring can be controlled.
Rzeppa joints are well known from various publications, for example U.S. Pat. No. 1,975,758; German utility model 8,402,784.3; French patent 849,676; German patent 889,851; French patent 1,497,696; German patent publication 1,183,318 and German patent application 3,636,243 and German patent publication 1,167,618.
It is also well known to use such a Rzeppa joint for coupling a cylinder barrel with the drive flange in axial piston type hydrostatic motors.
German patent publication 1,220,735 discloses an axial piston type motor, wherein the cylinder barrel is driven by the drive flange through a Rzeppa joint. In the motor of German patent publication 1,220,735, the outer joint member is a cup-shaped element provided on an axial projection of the cylinder barrel. The inner joint member is attached to a pin on the side of the drive flange. The drive flange has a central recess to accommodate the Rzeppa joint with its outer joint member. Ball-and-socket joints by which the piston rods are articulated to the drive flange are arranged in a circular array around this recess.
In the motor of German patent publication 1,220,735, the size of the Rzeppa joint is limited. This involves the risk that the Rzeppa joint is subjected to wear when transmitting the torques required to rotate the cylinder barrel. Such torques are mainly due to the frictional forces between the valving surface and the end face of the cylinder barrel and may become quite large, if the motor is operated with fluid under very high pressure. Increasing the dimensions of the Rzeppa joint results in an increase of the overall size of the whole motor. The Rzeppa joint may also interfere with the piston rods at large pivot angles and, thereby, limit the pivotal movement of the barrel support. Also off-center pivotal movement of the barrel support and of the cylinder barrel, as explained above, is not possible with the design of German patent publication 1,220,735.
German patent application 1,775,222 shows a hydrostatic transmission with an axial piston type motor, wherein the cylinder barrel is coupled with the drive flange by means of a Rzeppa joint. In this design, the outer joint member of the Rzeppa joint is arranged in a recess of the drive flange. The inner joint member is arranged on an axial projection of the cylinder barrel. The piston rods are articulated to the drive flange in a circular array around the recess and radially spaced therefrom.
German patent application also shows a valving body engaging with a valving surface the end face of the cylinder barrel. The valving body has valving ports in its valving surface for alternately connecting the cylinders of the cylinder barrel to a fluid inlet or a fluid outlet. This valving body is held in engagement with the end face of the cylinder barrel by a bolt extending centrally through the valving body. The bolt extends into a cavity within the cylinder barrel. A compression spring is located in this cavity and abuts, at on end, the outer race of a ball bearing retained at the end of the bolt and engages, at its other end, the end face of the cavity.
German patent application 3,522,716 discloses an axial piston type motor similar to that of German patent application 1,775,222. Also in this motor, the piston rods are articulated to the drive flange in a circular array radially spaced from the central recess, in which the Rzeppa joint is arranged. Piston rods and ball-and-socket joints are arranged in the same longitudinal planes as the grooves and balls of the Rzeppa joint. A central shaft, extending through the cylinder barrel is supported with a spherical surface on the spherical inner surface of the central recess of the drive flange.
German patent 941,246 shows pressure fields in the valving surface of an axial piston type motor.
German patent 1,051,602 shows a hydrostatic axial piston type motor, wherein arcuate grooves are provided radially outwards and radially inwards of the arcuate valving ports in the valving surface. These grooves are connected to the fluid inlet or to the fluid outlet through passages drilled in the valving body below the valving surface.